1. Field of the Invention
The present invention relates to a receiver in an ultra-wideband (UWB) wireless communications system.
2. Description of the Prior Art
UWB wireless communication, which uses a signal pulse width on the order of several nanoseconds, is attracting attention as wireless communication that can achieve high-speed data transmission with low power consumption. UWB wireless communication is susceptible to the effects of timing jitter arising on the receiving side, and there is a possibility that the transmission error rate may deteriorate greatly due to interference among pulse signals arising due to the use of large numbers of devices. Conceivable methods of improving this transmission error rate include the application of channel encoding. Up until now, Reed-Solomon encoding, convolutional encoding and turbo encoding have been studied as the channel encoding in UWB wireless communications, and convolutional encoding and turbo encoding have been shown to be effective.
Reference Document 1 (R. Herzog, A. Schmitbauer, and J. Hagenauer, “Iterative Decoding and Despreading Improves CDMA-Systems using M-ary Orthogonal Modulation and FEC,” IEEE International Conference on Communications, Montreal, Canada, pp. 909-913, June 1997.) reports an iterative decoding method of the constitution shown in FIG. 4. This reported decoding method is a constitution wherein the turbo principle presented in Reference Document 5 (J. Hagenauer, “The turbo principle: Tutorial introduction and state of the art,” International Symposium on Turbo Codes, Brest, France, pp. 1-11, September 1997.) is used as a decoding method for improving the transmission error rate in the IS-95(B) Code Division Multiple Access system using narrowband signals. In FIG. 4, the output from a Fast Hadamard Transformer (FHT) is sent to a channel decoder via a deinterleaver, and in the decoder decoding is performed based on the Maximum A posteriori Probability (MAP) decoding algorithm or the Soft Output Viterbi Algorithm (SOVA), and the output is fed back to the FHT as a priori information. Decoding of the input and output from each FHT and is performed using the log likelihood ratio which is soft information.
In addition, Reference Document 2 (S. ten Brink, “Iterative Decoding for Multicode CDMA,” IEEE Vehicular Technology Conference, Vol. 3, pp. 1876-1880, May 1999.) reports the iterative decoding method illustrated in FIG. 5. This reported decoding illustrates a method wherein iterative decoding is performed on the receiving side by using the turbo principle illustrated in Reference Document 5 in narrowband multicode CDMA where a plurality of spread codes are used to perform parallel transmission. Regarding the constitution of FIG. 5, the likelihood of information transmitted in each spread signal is calculated in the code demapper, and this likelihood information is sent to the decoder via a deinterleaver, and in the decoder, decoding is performed based on an a posteriori probability (APP) decoding algorithm, and the output is fed back to the code decoder as a priori information. Decoding of the input and output from each code demapper and decoder is performed using the log likelihood ratio which is soft information.
The conventional decoding methods described above are ones intended to improve the transmission error rate characteristics of iterative decoding on the receiver side when using the turbo principle in narrowband Code Division Multiple Access (CDMA). Each consists of a block that calculates the likelihood of transmitted spread codes along with a deinterleaver, decoder and interleaver, thus achieving iterative decoding by the feedback of likelihood information from the decoder via the interleaver.
In addition, Reference Document 3 (03154r2P802-15_TG3a Xtreme Spectrum CFP Presentation. Proposal for IEEE 802.15.3a. May 2003.) presents a study of the application of convolutional code or Reed-Solomon code to UWB wireless communication. On the receiving side, hard decisions are made on each pulse signal and decoding is performed on each code. Since hard decisions are made on the receiver side, the improvement of the transmission error rate characteristics becomes smaller than in the case of using iterative decoding based on a soft-input/soft-output algorithm.
In addition, Reference Document 4 (N. Yamamoto and T. Ohtsuki, “Adaptive internally turbo-coded ultra wideband-impulse radio (AITC-UWB-IR) system,” IEEE International Conference on Communications 2003, pp. 3535-3539, May 2003.) applies turbo codes to UWB wireless communications. As shown in FIG. 6, the receiving side consists of a pulse correlator, integrator and turbo decoder. In this method of Reference Document 4, recalculation of the likelihood information on received pulses is not performed using a priori information from the decoder.
The present invention has as its object to provide a receiver that is able to improve the transmission error rate in an arbitrarily encoded UWB wireless communications system.